1. Field of the Invention
The present invention relates to a photomask defect correction method employing a combined device of a focused electron beam device and an atomic force microscope.
2. Description of the Related Art
As fine detailing of patterns becomes smaller, photomask defects of an original plate of a pattern transfer for a wafer to be corrected also become small, and high-precision correction can also be obtained. Further, optical proximity correction (OPC) patterns that are even smaller patterns for correcting optical proximity results so as to increase resolving power have also been introduced which means that it is also necessary to correct defects of OPC patterns that are smaller than these patterns. In addition to defect correction devices employing lasers and focused electron beams of the related art, it is also possible to carry out scratch processing of physically eliminating opaque defects using a diamond tip that makes full use of the high resolving power and superior positional control of an atomic force microscope (for example, “Y. Morikawa, H. Kokubo, M. Nishiguchi, N. Hayashi, R. White, R. Bozak, and L. Terrill, Proc. of SPIE 5130 520-527(2003)).
With AFM scratch processing devices of the related art, low magnifications are observed using an optical microscope and high magnifications are observed using an AFM. There is still a disparity of 10 to 20 μm even if the mask alignment is carried out and the defect scanning device and defect coordinates are linked. It is therefore not possible to know the positions and shapes of defects even when an optical microscope is at maximum magnification because the defects are too small. Image acquisition therefore has to be carried out using an AFM image, which is time-consuming. It is also difficult to search for defects with a field of vision of 40 μm even with an AFM image. Observations are therefore carried out with a field of vision of 10 to 20 μm at positions where defects seem likely to exist and if defects are not found, the observations take place while changing the location, with scratch processing then being carried out upon finding defects. Finding the positions of the defects is therefore more time-consuming than the processing.
The diamond tip is worn after being used in processing and therefore has to be changed. When the tip is changed, it is necessary to always use a dedicated pattern in order to line up the center of the field of vision of the optical microscope and the center of the AFM field of vision. In addition, as described above, the center of the field of vision is offset by approximately 5 μm with respect to the center of the field of vision of the AFM because the magnification rate of the optical microscope is low even in the case of alignment of the centers of the fields of vision. Therefore, even if the centers of the fields of vision are lined up so that observation with a field of vision of 10 μm takes place once, observation then takes place again with a field of vision in the order of 2 μm from the defect being at the center. The load in this field of vision is then increased, and scratch processing is carried out.
Further, with AFM scratch processing, correction is carried out so that a surface eliminated using a special-shaped probe having a vertical cross-section for eliminating opaque defects connected to the pattern becomes vertical. It is therefore necessary to rotate the mask in the direction of the defect. Because of this, it is therefore necessary to carry out mask alignment so as to re-link the defect scanning device and defect coordinates so as to obtain an image and then perform a time-consuming search for defects using an AFM image, which is still more time-consuming.
In addition to opaque defects that are surplus to a pattern in a photomask, there also exist clear defects that are pattern deficiencies, and in theory, it is not possible for the AFM scratch processing device to correct clear defects. In this event, correction takes place using an FIB-CVD film or electron beam CVD film by employing a focused ion beam device (FIB) or electron beam device. However, various problems occur such as thin films of 0.1 μm or less not becoming attached, sagging occurring in the shapes of the films so that transmittance is lowered by a hollow component, and charging up occurring in order to correct a photomask constituted by an insulator so as to cause positional precision to fall. High-precision correction for defects of a level small enough to enable correction using an AFM scratch processor to clear defect correction are then obtained.
In addition to this, at the time of correction using an AFM scratch processing device, in the event that opaque defects are overcut, it is not possible to achieve anything by just using an AFM scratch processing device that can only perform elimination processing. In this case, correction of overcut portions takes place using a clear defect correction function of an FIB or electron beam device but high-precision correction of defects of a level small enough to enable correction using an AFM scratch processor was not possible. Further, in the related art, separate devices exist, AFM scratch processing devices operating in the atmosphere and FIB and electronic beam devices requiring a vacuum. Because of this, when both AFM scratch processing devices and FIB and electronic beam devices requiring a vacuum are employed, it is necessary to transfer a sample between each device. Only defect positions are required by respective devices where samples are transferred. In cases where it is necessary to alternately use AFM scratch processing devices and FIB and electron beam devices a plurality of times, the number of times it is necessary to output positions of defects using time-consuming AFM scratch processing increases, which causes operation time to increase substantially. Further, at the time of use of the FIB and electron beam device, time is required every time vacuuming takes place and to await the release of the atmosphere, re-alignment of the mask is necessary every time a mask is replaced, which is an extremely time-consuming operation.
In order to resolve the aforementioned problems, the present invention is advantageous in increasing throughput of AFM scratch processing, and enabling small clear defects to be corrected with a high degree of precision in clear defect correction. Further, in the event of over-removal in AFM scratch processing, correction is possible over a short time.